1. Field of the Invention
This invention relates to a heteroleptic iridium complex for an organic light-emitting diode, more particularly to a heteroleptic iridium complex having a dianionic bidentate ligand and used for phosphorescent organic light-emitting diode (OLED) devices.
2. Description of the Related Art
Organic electroluminescence devices have been applied in the fabrication of flat panel displays in recent years due to their spontaneous emission of light, high efficiency, energy-saving characteristics, low operation voltage, etc. An organic electroluminescence device normally includes an organic light-emitting diode (OLED) and a driving element. An OLED includes an anode, a cathode, and an organic layer disposed between the electrodes for generation of light when a voltage is applied between the anode and the cathode. Generally, the organic layer is made from a phosphorescent material, in which both the singlet and triplet excitons can be harvested and the internal quantum efficiency can reach as high as 100% by utilizing both of the generated excitons. As such, it is desirable to improve both the preparation and the fundamental characteristics of phosphors so as to enhance the light-emitting efficiency of OLEDs. Phosphorescent iridium complexes have been disclosed in, for example, US 2011/0282059 A1, and an article entitled “Homoleptic Tris (Pyridyl Pyrazolate) IrIII Complexes: En Route to Highly Efficient Phosphorescent OLEDs” (Chem. Eur. J. 2010, 16, 4315-4327). In these two documents, two classes of phosphorescent iridium complexes such as homoleptic and heteroleptic iridium complexes are discussed.
For example, a complex of the following formula (a) is a homoleptic iridium complex (formula EM-1 disclosed in US 2011/0282059 A1), and a complex of the following formula (b) is a heteroleptic iridium complex (formula EM-4 disclosed in US 2011/0282059 A1):


where,
Ra to Re are

and
respectively,
Rf is C1 to C20 alkyl or C1 to C20 alkoxy, and
Cy is a C1 to C4 carbocyclic ring, or a C1 to C4 heterocyclic ring.
In the homoleptic iridium complex of formula (a), all the ligands are the same, and thus, it is difficult to fine-tune the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and hence, tuning of the emission wavelength. The heteroleptic iridium complex of formula (b) is an ionic iridium complex which is liable to be left on the silica gel column during a separation process (i.e., silica gel column chromatography) and thus has a low product yield. In addition, the ionic iridium complex has poor volatility so that the organic layer cannot be formed by a conventional vacuum deposition process, and needs to be formed by a solution (or wet-printing) process which may render the light-emitting efficiency of the OLED lower.